If the law has specific clauses about this that the contract disrespects, these conditions are not worth the paper they are written on.

At least in Brazil you can't enforce something the law doesn't allow in a contract - that clause would be considered void without nullifying the contract. And Labour law in Brazil leans (or used to lean) more in favor of the employee,so yes, the law would win. Another aspect there is that unions are more common than in the US, and they will help in such cases.

>If the law has specific clauses about this that the contract disrespects, these conditions are not worth the paper they are written on

Unless the law also has severe penalty for including such terms, of course they are. They don't need to dissuade 100% of people from breaking 100% of the terms to be of use to the company.